The present invention relates to a compacting process suitable for storing with safety over a prolonged period of time radioactive metal wastes such as spent nuclear fuel cladding (hereinafter referred to as "hulls") which are sheared into short lengths for nuclear fuel reprocessing.
In compacting and stabilizing such radioactive metal waste for storage of the waste, attention has been directed in recent years to a hot isostatic pressing (hereinafter referred to as "HIP") treatment.
For example, Examined Japanese Patent Publication SHO No. 57-959 discloses a process comprising precompressing radioactive metal waste in a die to obtain a block, filling the block into an HIP treatment container, degassing or non-degassing and sealing the HIP treatment container, and subjecting the sealed HIP treatment container to HIP treatment in its entirety. This process includes the precompressing step in order to give an increased bulk density (before the HIP treatment) to the radioactive metal waste to be filled into the HIP treatment container. The increased bulk density serves to minimize the deformation of the HIP treatment container by the HIP treatment and avoid a break of the HIP treatment container that would occur if the HIP treatment container is deformed greatly. From this viewpoint, it is desired that the precompressed (press-formed) block have a bulk density of at least 60% of true density (waste metallic density).
With the process disclosed in the above publication, the compressed block to be filled into the HIP treatment container has an outside diameter slightly smaller than the inside diameter of the HIP treatment container and is approximately equal to the HIP treatment container in cross sectional area. Thus, the waste is precompressed with a die which is approximately equal to the HIP treatment container in cross sectional area.
A considerably great total pressing load is required when the desired bulk density/true density ratio is to be obtained with such a die which is generally equal to the HIP treatment container in cross sectional area. For example, for a die 300 mm in outside diameter to achieve a bulk/true density ratio of 65%, the total pressing load needed is as great as 1400 t. The precompressing step of the above process therefore requires the use of a heavy embossing machine which is great in total pressing load and which necessitates an increased space for installation and is unfavorable in respect of the cost.
Further with the above process, the compressed block is filled directly into the HIP treatment container without using another container or the like, with the result that the radioactive substance adhering to the waste is likely to scatter about or the waste will release small fragments during the filling procedure. The process has another problem in that since the waste is filled into the die directly for precompression, the waste comes into direct frictional contact with the die to cause marked damage to the inner surface of the die.